Hard-wired local area networks (LANs) have developed significantly since the inception of the Ethernet over two decades ago. Today, hard-wired LANs which are relatively inexpensive, fast, and reliable are widely available for both private and public applications. Businesses, educational institutions, governmental and other public entities have all significantly benefited from the ability of hard-wired LANs to electronically exchange information between physically separated network terminals. However, hard-wired LANs are clearly subject to the constraints imposed by the required physical infrastructure, and, in particular, to limitations on the mobility of individual network terminals as a consequence of the immediate availability or unavailability of the appropriate physical interconnect.
In view of the limitations on hard-wired LANs, as well as significant advances in wireless communication technologies, the development of standardized wireless LANs (WLANs) has been the next logical step. One particular standard which defines protocols for WLANs is ANSI/IEEE Std. 802.11 Wireless LAN Medium Access Control and Physical Layer Specifications. This standard is one of the ANSI/IEEE 802 family of standards applicable to both local and metropolitan area networks. Additionally, ANSI/IEEE Std. 802.11 is associated with a number of released, and proposed (draft) supplemental standards, including draft supplemental standard ANSI/IEEE Std. 802.11e MAC Enhancements for Quality of Service.
Quality of service (QoS) considerations are critical for supporting multimedia (audio/video) applications which require increased control over transmission rates, latency and error rates. QoS over a WLAN presents considerable challenges since changes in the wireless medium may significantly affect each of these parameters and consequently the quality of the presentation at the receiving terminal. In addition, different data categories require different priorities and different transmission strategies. For example, voice transmissions are more tolerant to latencies and data errors than multimedia transmissions in which data errors and latency are able to impede generation of a smooth, high quality presentation. On the other hand, text and relatively static display data have an even higher tolerance to errors and latency and therefore typically do not require the utilization of complex QoS mechanisms.
Even in view of draft supplementary standard ANSI/IEEE Std. 802.11e, many problems remain to be resolved with respect to the actual implementation of QoS principles in WLAN systems. Addressing these problems in a cost efficient manner has become a priority in the WLAN industry since consumers increasingly expect information processing systems to both support high-quality multimedia applications and provide the higher degree of convenience afforded by wireless communications.
One problem that is often encountered in wireless communication systems, such as wireless LANs and cellular telephony, is multipath fading. Multipath fading occurs when a signal is transmitted in an environment including one or more reflectors, which cause the signal to take multiple paths from the transmitting node to the receiving node. The direct signal from the transmitting node to the receiving node and each of the reflected signals normally differ in phase and/or amplitude when reaching the receiving node, and therefore constructively and destructively combine resulting in distortion. The problem of multipath fading is compounded when reflective or absorptive objects within the transmission environment and/or one or more of the transmitting and receiving nodes themselves are moving.